Field of the Invention
The present invention relates to a process for desulfurizing combustion exhaust gases containing sulfur dioxide (SO.sub.2), hydrogen fluoride (HF) and dust, and more particularly it relates to a process for preventing the reaction of limestone or slaked lime employed as an absorbent therefor with sulfur dioxide, from hindrance of HF and dust.
Recently coal has been used as a fuel for boilers of power stations or factories, because of the changes in the energy situation. Combustion exhaust gases of coal contain HF, dust and hydrogen hydrochloride (HCl), each in a concentration of about ten times those of combustion exhaust gases of heavy oil, and these components have various effects upon the desulfurization reaction. It has been known that among them, such metallic components as aluminum, contained in the dust have undesirable effects such that they inhibit the growth of gypsum crystal; they mix in gypsum as impurities to reduce the purity of gypsum; etc. Further, since HCl reacts with an absorbent of sulfur oxide such as quick lime to form calcium chloride (CACl.sub.2), it increases the amount of absorbent used. Furthermore, since CaCl.sub.2 reduces the dissolution rate of absorbent, it is necessary that the reaction time (retention time of slurry in the absorption step) is prolonged i.e. the capacity of slurry tank in the absorption step is increased in order to complete the desulfurization reaction.
Heretofore, there have been proposed various processes for solving these problems, such as a process of neutralizing metallic components to remove them in the form of floc of their hydroxides for the purpose of obtaining gypsum of a good quality (Japanese Patent Publication No. 10838/1977), and a process of adding magnesium salts to a slurry of absorbent for the purpose of preventing hindrance caused by HCl (Japanese Patent Application Laid-Open No. 17565/1978). Further a process of allowing dust not to be mixed in the absorption step by removing it prior to the absorption step, i.e. a so-called dust separation process, has been also proposed. This process, however, has a drawback that the apparatus therefor is corroded in the dust-removing step since the pH of the solution employed comes to be 2 or lower.
Further, the disposal of the dust thus separated also raises a problem.